It is well known that power in hydraulic systems is transmitted and controlled by pressurized fluid such as in the use of positive displacement pumps to convert shaft rotation into hydraulic power. Pumping frequency and the harmonics of such processes necessarily create variations in both the flow and pressure being carried by the fluid. These variations cause fluid-borne and structure-borne vibrations to be transmitted throughout the hydraulic system, including the plumbing system associated therewith. These fluid vibrations act as excitation drivers of the system components and become audible (air-borne) noise as vibrations of the component surfaces are transmitted to the surrounding air.
Pressure variation caused by flow variation frequently creates pump (fluid) noise, which becomes more prominent as pressure variation amplitude and frequency increase. Such pump-produced variations or "ripples" in pressure and flow are transmitted through the working fluid as fluid-borne noise which excites the surface of fluid conduits causing airborne noise and providing energy which may in turn excite any structural member or surface to which the conduit is attached. Variations in flow caused by a positive displacement pump are due to periodic variations in geometric displacement and fluid compression and expansion processes at the points of transition between high pressure and low-pressure elements of the system.
Geometric displacement variation, or the ripple effect discussed above, occurs because total flow is a summation of flow from the individual pumping elements. In the case of a piston pump, geometric flow varies as the sum of a series of half sine waves, amplitude of the flow ripple being dependent on the number of pumping elements. Also, the fundamental flow variation frequency for piston pumps matches the first order piston pass frequency. However, in most mobile machines, piston pass frequency varies with machine engine speed (rpm), and because engine speed varies over a wide range, piston pass frequencies will also vary over a wide range. Furthermore, when there is an odd number of pistons, the dominant frequency is normally twice the piston pass frequency.
It is therefore desirable to keep the rate of change in flow as low as possible, avoiding surges of fluid, in order to provide a smooth variation in flow. Avoiding a large amount of fluid flow change minimizes differences in amplitudes of the harmonics of the fundamental frequency. Such minimization in rate of change of flow variation can be controlled by properly controlling the timing of the inlet and outlet ports. Because timing of port (orifice) operation influences a number of other pump characteristics and must accommodate a wide variety of operating conditions, the design of port timing is usually a compromise, which can interfere with obtaining the optimum flow variation for a particular system.
A variety of approaches have been taken to address audible noise attenuation in hydraulic systems where the audible noise is the end result of fluid noise. When attached as a side branch to a pump outlet line, gas-charged accumulators can be used to reduce pulsations. However, they tend to be less effective than flow-through types of accumulators. Accumulators generally are low frequency devices which act to reduce the low frequency components of the pulsations with little effect on the critical mid-frequency components. At high frequencies, intervals between pulsations are so short that there is insufficient time for fluid to enter and exit the accumulator before the next pulse arrives. In such cases, some pulsations bypass the accumulator completely. Construction of a flow-through device can overcome this problem. Flow-through type accumulators are effective at almost all frequencies, however they tend to be bulky and expensive. In addition, gas-charged type accumulators require maintenance (charging to the correct pressure) and such accumulators are temperature sensitive. These limitations prevent gas-charged accumulators from being suitable for use on variable pressure systems.
Other known approaches to fluid noise reduction include the addition of a Helmholz resonator to a hydraulic system. This system requires providing a volume in a side branch of the system. This is accomplished by providing a fluid vessel generally adjacent the pump that has a predetermined length with a flow volume which can absorb and release fluid as the flow variation from the pump tries to suddenly increase and decrease flow through the flow restrictor that is located downstream thereof. Fluid in the volume and its connecting line forms a resonant subsystem in which output pressure pulsations and steady state pressure losses of the Helmholz resonator type muffler are minimal. This provides a more constant flow rate downstream. Thus, a Helmholz resonator can reasonably attenuate pressure pulsations from a hydraulic pump and can have a convenient small volume and simple structure. However, current Helmholz resonators are limited in that they have a fixed, non-adjustable volume and therefore can only provide high attenuation in narrow bands.
Another known approach to the fluid noise problem is the so called Quinke Tube which is an arrangement that splits flow equally between two lines of different lengths and recombines them at a downstream junction. The Quinke device works by effectively canceling the fundamental flow variation frequency and its harmonics because the two flows are out of phase when they merge since they travel different distances. Like the Helmholz resonator, known Quinke tubes have a fixed length and therefore can only provide high attenuation in narrow bands.
A further known approach to the problem of fluid noise being converted to audible noise has been the use of acoustic filters that work like an automotive muffler. Sometimes referred to as "tuned" filters, acoustic filters are as effective as flow-through, gas-loaded accumulators and do not require maintenance. However, acoustic filters must be selected on the basis of pumping frequency and flow capacity. Further, such filters can be bulky and expensive, and because they are also good sound radiators, they must sometimes be wrapped with noise insulation material so that any sound emissions emanating therefrom will not exceed the other reductions that will result from their use. In addition, some tuned filters can only provide high attenuation in narrow bands. Although tuned filters are sometimes used with a combination of other devices to broaden the tuning over a wider range, combining devices necessarily increases the bulk and cost of the entire system.
It is therefore desirable, to provide a system that is effective to control the fluid borne noise within a hydraulic system when operating at different speeds, different pressures, and/or different displacements.
Accordingly, the present invention is directed to overcoming one or more of the problems as set forth above.